Certain gases have absorption bands in the electromagnetic spectrum that absorb so weakly that absorption can only be detected after the radiation has traveled a relatively long distance, perhaps as long as tens of meters, through the gas. In other situations the gases of interest might have adequate absorption strengths but they must be detected in very low concentrations, typically in the parts-per-million (ppm) or even in the parts-per-billion (ppb) ranges, so that long path lengths are also required in this situation. But the most challenging situation occurs when the very low concentration of a weakly absorbing gas is required to be determined. Under this circumstance the need for a very long path-length sample chamber is absolutely indispensable.
Although numerous gas detection methodologies have been developed over the years, the most notable ones of which include electrochemical fuel cells, Figaro or Tin Oxide (SnO2) sensors, Metal oxide semiconductor (MOS) sensors, Catalytic (Platinum bead) sensors, Photo-ionization detector (PID), Flame-ionization detector (FID), Thermal Conductivity sensors etc., they are all commonly referred to as “interactive” types of gas detectors. As such almost every one of them without exception suffers from long-term drifts and non-specificity problems, particularly when applied to the detection of volatile organic compounds or VOC's. The Non-Dispersive Infrared (NDIR) technique, on the other hand, has long been considered as one of the best methods of gas measurement since the 1950s. In addition to being highly specific, NDIR gas analyzers are also very sensitive, stable, rugged, reliable, and easy to maintain. The advent of the so-called “wave-guide” sample chamber disclosed in U.S. Pat. No. 5,163,332 by Wong in 1992 and commonly referred to as simply “The Tube” significantly simplifies today's NDIR gas analyzers into compact, rugged and low-cost sensors while still maintaining their superior performance characteristics.
An even more superior gas measurement technique, which only became fully developed within the past two decades, uses the output of a laser as a coherent radiation source. It is called the Tunable Diode Laser Absorption Spectroscopy or TDLAS technique. TDLAS gas sensors in general have better gas detection sensitivity and are more compact and reliable than the corresponding NDIR counterparts. However, due to available wavelength restrictions, they have to depend upon the much weaker higher harmonics of the fundamental absorption bands of gases that they detect. Consequently, TDLAS sensors routinely require many times the path lengths required for their NDIR counterparts in order to attain the same detection sensitivities.
Practical gas analyzers for commercial use seldom incorporate very long path-length sample cells for such purposes primarily because of size limitations. Where the use of long path-length sample cells cannot be avoided, it is common to use lenses and mirrors to fold an optical beam so that the latter can traverse the sample cell a number of times in order to increase its effective path length.
A multi-path absorption cell was described by J. U. White, Journal of Optical Society of America, Volume 32, page 285 (1942). The essential parts of the White cell consist of three spherical concave mirrors all having the same radius of curvature and positioned to form an optical cavity. Over the years, utilizing the principle outlined in White's article, multi-path cells having path-lengths greater than 40 meters have been successfully constructed and even made available for public purchase. However, since the settings of the White cell has to be carefully and meticulously adjusted every time before its use in order for it to correctly provide the desired path-length, the use of White cells has been confined mainly to laboratories and they are seldom deployed for the construct of practical and transportable gas analyzers.
Another multi-path absorption cell was described by D. Herriott et al., Journal of Applied Optics, Volume 3, page 523 (1964). The Herriott cell consists of two spherical mirrors separated by nearly their radius of curvature. The optical beam is to be injected through a hole in one mirror and is reflected back and forth a number of times before exiting from the same hole. Unlike the White cell, the beam remains essentially collimated throughout its traversals of the cell. Although the Herriott cell is in principle simpler to align than the White cell since it consists of only two mirrors, the number of passes through the cell is determined by the exact mirror separation which has also to be meticulously adjusted every time before its use. Thus, like the White cell, the Herriott cell is also not robust enough for use in portable or transportable gas analyzers.
In U.S. Pat. No. 4,756,622, Wong in 1988 disclosed another approach for providing long path-lengths for measuring the weak absorption by a gas. In Wong's invention, light is made to travel through a limited volume of the gas a large number of times. The light is placed on a closed optical path on which it circulates through the gas sample. After a desired number of passes through the sample, the light is removed from the closed optical path. Introduction of the light to the closed optical path and removal therefrom is accomplished through the use of a polarizing beam-splitter and a Pockels cell located on the closed path. Light is put onto the closed path by the polarizing beam-splitter which imparts a specific polarization. During the first circuit the Pockels cell alters the polarization by 45 degrees thereby preventing the light from escaping back out through the polarizing beam-splitter. After a desired number of passes, the Pockels cell again alters the polarization by 45 degrees thereby permitting the light to be redirected out of the closed path by the polarizing beam-splitter. In this way the so-called “Wong cell” can readily achieve very long path-lengths of hundreds or even thousands of meters or more.
While the long path-length sample chamber disclosed by Wong in U.S. Pat. No. 4,756,622 is indeed compact and does not require careful and painstaking adjustments of its optical components prior to its use, it does require a rather complex and expensive component, namely the Pockels cell and its associated electronic driver, in order to make the overall scheme functional. A typical Pockels cell system comprising the cell and its electronic driver costs upwards of $10,000.00 (for example Pockels Cell System Model No. 5046 manufactured by Lasermetrics, Div. of FastPulse Technology, Inc.). Furthermore, The Pockels cell is a wavelength dependent device requiring a number of electro-optical crystals (e.g. KDP for wavelengths up to 1.33 microns and Lithium Niobate for up to 1.6-1.7 microns etc.) for its construct. Consequently, and unlike the White and Herriott cells which are wavelength independent, the use of the Wong cell is often times limited only to those instances when the appropriate Pockels cells are available with the right electro-optical crystals and having the correct optical transmission in the electromagnetic spectrum for its use.
The White and Herriott cells described above require meticulous settings prior to their uses whereas the Wong cell (U.S. Pat. No. 4,756,622 (1988)) suffers from the disadvantages of being too expensive and wavelength dependent applications due to the limited availability of suitable electro-optical materials for making the appropriate Pockels cells. Nonetheless, as the world's population continues to grow, particularly in urban areas, and the people's living environments get progressively worsened as a result of more energy consumption through the burning of more and more fossil fuels, the need to measure weak absorbing toxic gas species in very low concentrations (ppb's) such as Carbon Monoxide (CO), Nitrogen Dioxide (NO2), Ozone, volatile organic compounds (VOC's) etc., in both open and closed spaces in order to protect public's health, becomes ever more pressing. The tragic 9/11 terror attack incident exacerbated the situation even more. In order to prevent future terror attacks on the masses using extremely toxic gases such as mustard gas, sarin and other deadly nerve agents, the need to detect weakly absorbing lethal gases in the minutest amount (parts per trillion or ppt levels) accentuates the situation even more urgently than ever before. The complex and expensive long path length sample chambers proposed by various authors discussed above are clearly inadequate for use as portable and transportable gas analyzers for this type of application. Simple, rugged, low-cost and long path-length sample cells, suitable for use with both the NDIR and TDLAS type transportable gas analyzers, are, to the best of the present inventor's knowledge, not available commercially today even though they are very urgently needed. It is to this long felt need that the present invention is directed.